I. Field of the Invention
The field of the invention is directed to a rack bearing assembly for a yoke apparatus for use with a rack-and-pinion steering system.
II. Description of the Prior Art
Automotive steering systems typically include a housing having a rack driven by a pinion gear. Rotation of a steering wheel turns the pinion gear. The pinion gear meshes with a plurality of teeth formed on the rack to drive the rack in one of two reciprocal directions. The rack in turn is connected to a pair of dirigible wheels. In addition, many automotive steering systems comprise a rotary control valve which is operable to supply pressurized fluid to move a double-acting hydraulic cylinder or actuator to assist translation of the rack.
In order to keep the teeth of the pinion gear teeth and the teeth of the rack in engagement, conventional steering systems employ a yoke apparatus. The yoke apparatus includes a rack bearing which is biased to force the rack towards the pinion gear. The rack bearing has a pair of spaced apart bearing surfaces which slidingly contact the surface of the rack opposite the teeth of the rack. The rack bearing is mounted in a bore in the housing which is formed in a nominally orthogonal manner with reference to the housing in an effort to align the bearing surfaces along an axis which extends coaxially with the nominal axis of translation of the rack. A spring is mounted in the bore to force the yoke assembly against the rack and bias the bearing surfaces in order to force the teeth of the rack against the teeth of the pinion gear. Thus, the yoke apparatus operates to nominally guide the rack along the axis of translation and hold the teeth of the rack and pinion in mesh during the application of torque to the pinion gear.
In practice, it is not possible to maintain the axis of translation of the rack orthogonal to the axis of the bore. This is because of the tolerances involved in forming the bore, rack and pinion gear. Accordingly, it has been found that the axis of translation of the rack may be angled with respect to the axis of the bearing surfaces of the rack bearing, and may even undulate as a function of rotational motion of the pinion. When so misaligned, one end of each of the support surfaces engage the rack while opposite ends of the support surfaces are spaced away from the rack. As a further result, the rack bearing itself may suffer angular misalignment within the bore and jam. In fact, such yoke assemblies may be said to be over constrained or to be of non-Kinematic design.
For the above reasons, the rack is often held from smooth movement in one, or both, directions of travel. This is particularly so when the rack travels in a direction from the contacting ends towards the non-contacting ends of the support surface. The edges resist movement of the rack and the rack tends to hesitate and jerk in its movement. However, movement of the rack in an opposite direction tends to produce a smoother, less resistant movement. Frequently, the discontinuous or halting movement of the rack will be tactilly sensed by the driver.
The spring is located in an adjuster plug which is threadably inserted in the outer portion of the bore. During assembly of the yoke apparatus, the adjuster plug is rotatably driven into contact with the rack bearing with a nominal torque value of perhaps 50 in.lbs. to provide a rotational position calibration. Because of the above noted tolerances involved in forming the bore, rack and pinion gear, there results a soft contact between the adjuster plug and the rack bearing, and thus an imprecise rotational position calibration. For this reason, the adjuster plug must then be backed off by an angle of about 30 degrees in order to ensure interference free operation in the manner described above. This results in an indefinite stop position of the rack bearing should a torque level be applied that is sufficient to overcome the spring bias.
Because of the possibility of excessive operating clearance in the mesh between pinion gear and rack teeth, a spring strong enough to guarantee full engagement of pinion gear and rack teeth for all levels of applied steering wheel torque is utilized. This results in excessive Coulomb friction both with reference to the mesh of the pinion gear and rack teeth and contact of the bearing surfaces and the rack. The Coulomb friction results in hysteresis in the relationship between applied torque and rack motion. The result is a reluctance of such rack-and-pinion steering systems to return to center. This is true for both manual and power assisted steering systems and is one of the causes of imprecise "on-center feel".
A mechanism for correcting the above described deficiencies is found in the prior art in U.S. Pat. No. 3,680,443 entitled "Steering Mechanism for Motor Vehicles" by Leslie Richard Jenvey of Reading, England. In U.S. Pat. No. 3,680,443, a bifurcated yoke mechanism described therein as pad (13) and comprising upper and lower yoke members is shown but not fully described. The interface between the upper and lower yoke members of pad (13) is pictured as being the mating of a convex spherical surface on the upper yoke member with a concave spherical surface on the lower yoke member. In the case of the overall steering mechanism shown in U.S. Pat. No. 3,680,443, a pin (3) is said to allow for limited lateral motion of a rack (1) by a sliding motion thereof along the pin (3). In that condition, it would appear that the mechanism comprising the rack (1) and the upper and lower members of pad (13) is under constrained and therefore inoperative. This is because the lateral position for the rack (1) is undefined. In fact, as will be presented below, each of both the rack (1) and upper member of pad (13) have only five positional constraints.
Jenvey discloses permitting the rack to slide along pin (3) in a direction orthogonal to the axis of the rack between the bores 9 and 10. However, such a device is not satisfactory for use with modem rack and pinion steering gear. Modern steering gear utilizes positional constraints to maintain the position of the rack. These constraints include a plain bearing located remotely from the pinion. The plain bearing provides pitch and yaw constraints to the rack. The pinion provides axial location as well as roll and elevation position constraints. The yoke provides the remaining lateral constraint in the transverse direction, as well as compressive loading in the elevation direction. Thus, the yoke mechanism disclosed by Jenvey would be inoperable in a modern rack and pinion steering gear because it permits the rack to move laterally and does not provide the required lateral constraint. Accordingly, it is desirable to provide a yoke mechanism which overcomes the problems mentioned above, as well as provide lateral constraint.